Slope Facies Research Articles

Microbial reefs in eastern Yangtze Platform, South China Block: the last golden age of stromatolites in the Ordovician

In the eastern Yangtze Platform of South China Block, Tremadocian (Early Ordovician) stromatolites in southern Anhui Province occur along a palaeoenvironmental transect from the shallowest intra-platform setting through to deeper platform margin. The intra-platform and platform margin belts of Tremadocian rocks in the eastern platform are carbonate-dominated marine environments favourable for calcimicrobes. However, stromatolites are absent in siliciclastic-dominated slope facies. Shallower carbonate facies are represented by the upper member of the Lunshan Formation, composed of a hundred metres thickness of stromatolite-bearing carbonates. Such stromatolites at the Beigong section of Jinxian county are characterized by stratiform and dome-shaped laminations. Girvanella filaments are ubiquitous in thin sections. Crinoids grew on the surfaces of the microbialites. In contrast, towards the deeper carbonate platform margin, stromatolites in massive-bedded limestones of the upper member of Lunshan Formation of the Ma’anshan section at Shitai occur as dense columns of bindstones; macrofossils and bioclasts are rare in those stromatolites, indicating a lower energy and deeper marine setting. Water depth at the platform margin of the Ma’anshan section is interpreted as being above the base of the euphotic zone favorable for photosynthesis in the calcimicrobial community; however, benthic fauna notably declined. In contrast to shallower settings, in clastic-dominated locations interpreted as deposited in deeper water, stromatolites are absent, represented by the middle member of Dawuqian Formation at the Ziliqian section. This work, therefore, supports that Early Ordovician reef systems were still microbial-dominated, representing the last golden age of stromatolites.

Lithology and hydrocarbon potential of the paleozoic sediments of the
 North Caspian uplift (northern part of the Caspian sea, Kazakhstan)

The North Caspian uplift is located in the water area of the Caspian Sea and coincides with the area of articulation of the Caspian Depression and the Turan Plate. Drilling of wells proved the presence of a large carbonate platform in the North Caspian uplift composed of Middle Carboniferous deposits, which is similar in structure to the South Emba and Zhanazhol zones of the Caspian Depression. Carbonate deposits of the Lower Permian, formed in shallow-shelf, bioherm and slope facies were discovered in the North Caspian uplift.

Facies and geometry of drowning steps in a Miocene carbonate platform (Maldives)

The study of the Miocene carbonate platform of the Maldives allows understanding the controlling factors triggering the stepwise drowning of carbonate platforms. This research presents high-resolution seismic profiles and sediment cores retrieved from Sites U1465, U1469, and U1470 drilled during International Ocean Discovery Program (IODP) Expedition 359. A microfacies analysis of the platform deposits was conducted and correlated with a sequence stratigraphic analysis of seismic reflection data. The Neogene evolution of the Maldives encompasses a series of drowning steps. Two of these steps are compared with regard to the carbonate factory type to unravel if fossil assemblage surviving the first drowning step are different from younger assemblages. In both cases the shallow water carbonate facies are formed by coral-coralline algal boundstone and foralgal floatstone that change basinwards into slope facies dominated by large benthic foraminifera grainstone to packstone, locally floatstone and rudstone. The red algae and large benthic foraminifera assemblages are similar before and after the first drowning step. After this first drowning, the carbonate platform developed under enhanced bottom current conditions, as deduced by the coeval drift deposits. The carbonate platform and the drift deposits were balanced for circa 2 Myr, until a global sea-level fall at 10.6 Ma and later reflooding of the platform, which destroyed the equilibrium between both depositional systems, favouring the current-controlled deposits. The drowning of the Miocene carbonate platform of the Maldives was the result of the combined effect of the fluctuating sea-level during the Neogene and the intensification of oceanic currents due to the invigoration of the South Asian Monsoon during the middle Miocene.

Effects of Fe oxides on organic carbon variation in the evolution of clayey aquitard and environmental significance

As an important part of groundwater systems, aquitards may have a considerable impact on the quality of groundwater in an aquifer. Organic carbon (OC) serves as an important component in biogeochemical processes which affects the hydrochemical composition of pore water; the association of OC with (Iron) Fe-containing minerals is recognized as an important stabilization mechanism for OC. However, the characteristics of the process forming complexes between OC and Fe oxides in subsurface aquitards and the contribution of OC to aquifers has been poorly understood. In this study, the content, speciation and reaction types of OC and Fe oxides were investigated to reveal the characteristics and mechanisms of OC-Fe interaction in subsurface aquitards at different depths in two typical sedimentary facies of the Jianghan Plain in central China. The total organic carbon (TOC) contents of aquitard sediments in alluvial-lacustrine and residual slope facies were 7.25 ± 5.10 and 2.79 ± 0.86 mg g−1, respectively. In general, the proportion of the heavy fraction of OC (HFOC) to TOC gradually increased with increasing depth because consumption of the light fraction of OC (LFOC) that caused more HFOC to remain in sediment. On average, Fe bound-OC contributed 30.3% and 31.6% of TOC by adsorption and coprecipitation, respectively. The adsorption-stabilized OC has not changed obviously but the coprecipitation-stabilized OC increased gradually with depth. Coprecipitation stabilized more OC in a stable environment (residual slope facies) when compared with an unstable environment (alluvial-lacustrine facies), which can be supported by the greater average ratio of FePP bound-OC:TOC ratio and increased enrichment of carboxylates and aromatics in a stable environment. The transformation of OC-Fe complexes could affect the transport of As, Cr and ammonium which chemically bind to organic matter (OM) and Fe minerals from sediment to pore water by reductive dissolution. The findings of this study are helpful in understanding the interaction between OC and Fe oxides in subsurface aquitard environments, in which reaction products may affect adjacent aquifers.

New conodont records of the Los Sombreros Formation (Cambrian–Ordovician) from the Western Precordillera, Argentina: biostratigraphic and palaeoenvironmental implications

AbstractThe presence of a carbonate platform that interfingers towards the west with slope facies allows for the identification of an ancient lower Palaeozoic continental margin in the Western Precordillera of Argentina. The Los Sombreros Formation is essential for the interpretation of the continental slope of the Precordillera, which accreted to Gondwana as part of the Cuyania Terrane in the early Palaeozoic. The age of these slope deposits is controversial; therefore, a precise biostratigraphic scheme is critical to reveal the evolution of the South American continental margin of Gondwana. The study of lithic deposits of two sections of the Los Sombreros Formation, the El Salto and Los Túneles sections, provides important information for further understanding the depositional history of the slope. At El Salto section, the conodonts recovered from an allochthonous block refer to theCordylodus proavusZone (upper Furongian). The conodonts recovered from the matrix of a calclithite bed of the Los Sombreros Formation in the Los Túneles section are assigned to theLenodus variabilisZone (early Darriwilian), providing a minimum age for this stratigraphic unit. In addition, clasts from this sample yielded conodonts from thePaltodus deltifer−Macerodus dianaezones (upper Tremadocian). The contrasting conodont colour alterations and preservation states from the elements of two latter records, coming from the same sample, argue the reworked clasts originated in the carbonate platform and later transported to the slope during the accretion process of the Precordilleran Terrane to the South American Gondwanan margin during the Middle–Late Ordovician.

Evolution of an Archean fan delta and its implications for the initiation of uplift and deformation in the Barberton greenstone belt, South Africa

ABSTRACT The 3.28 to 3.23 Ga Mapepe Formation in the Barberton greenstone belt, South Africa, marks the initiation of widespread tectonic uplift and deformation after nearly 300 million years of predominantly basaltic and komatiitic magmatism under largely anorogenic conditions. This rapid transition is recorded in the eastern Barite Valley area by the buildup of a fan delta. Well-exposed sections there reach about 450 m thick and can be divided (from base to top) into five informal members: Member 1 is dominated by mudstone with subordinate banded ferruginous chert and turbiditic sandstone representing a deep-water basinal environment. Member 2 is composed of siltstone and fine-grained sandstone reworked by currents to form laminated, cross-laminated, and low-angle cross-stratified sediments in an off-shore or possibly subtidal fan-delta-front setting. Member 3 overlies member 2 unconformably; it is composed of predominantly coarse-grained, cross-bedded sandstone interbedded with laminated mudstone deposited on shallow-subtidal to intertidal flats along the fringe of a small fan delta in which putative microbial mats covered low-energy upper tidal flats. Fan-delta sedimentation was subsequently overwhelmed by the influx of dacitic pyroclastic sediments of member 4. Orthochemical sedimentary rocks including barite, jaspilite, and chert deposited on top of this shallow-water bank. Mappable facies changes towards the northeast and southwest document the transition from bank top into major mass-transport deposits of fan-delta slope facies and then into basinal deposits. Subsequent relative sea-level rise resulted in the return to below-wave-base deposition of turbiditic sandstone, mudstone, and banded ferruginous chert of member 5. The lenticular geometry of units in cross section, mineralogical immaturity, and high variability in provenance of the coarse-grained units imply short-distance transport of sediment derived from strata of the underlying Onverwacht Group and from local penecontemporaneous dacitic volcanism. Throughout the greenstone belt, Mapepe rocks in several structural belts display fan deltas developed adjacent to small, local uplifts. While the cause of these uplifts has generally been associated with the initiation of geodynamically driven tectonic activity in the BGB, it is possible that a cluster of large meteorite impacts may have directly or indirectly triggered the crustal deformation.

Carbonate carbon isotope chemostratigraphy and U-Pb zircon geochronology of the Liuchapo Formation in South China: Constraints on the Ediacaran-Cambrian boundary in deep-water sequences

The Ediacaran-Cambrian transition was one of the most significant intervals in geological history, witnessing the origin of the metazoans and major environmental changes. Establishing a precise chronostratigraphic framework for the Ediacaran-Cambrian transition has great significance for understanding the geological and evolutionary processes during this transition. An integrated study on carbonate carbon isotopes and U-Pb zircon dating was conducted within the Liuchapo Formation in the Sifangjing section in northeastern Guizhou Province, South China to recognize the Ediacaran-Cambrian boundary in slope facies of the Yangtze Block. A substantial negative carbonate carbon isotope excursion was recognized within the lower part of the Liuchapo Formation. Moreover, a weighted-mean U-Pb zircon age of 540.7 ± 3.8 (±6.6) Ma was obtained from a tuffaceous interlayer approximately 2 m above the interval of the negative carbon isotope excursion. The carbon isotope and age-dating results indicate that the Ediacaran-Cambrian boundary within the Sifangjing section should be placed in the lower part of the Liuchapo Formation. Additionally, in the Sifangjing section, a distribution of Horodyskia minor extends up to the basal Cambrian, while the last appearance datum (LAD) of Palaeopascichnus jiumenensis correlates to the large negative carbon isotope excursion within the Liuchapo Formation. This relationship indicates that the LAD of the Palaeopascichnus jiumenensis is a potential tool for the stratigraphic correlation of the Ediacaran-Cambrian boundary successions in slope and deep-water basin settings of the Yangtze Block.

Composition, Provenance, and Tectonic Setting of the Southern Kangurtag Accretionary Complex in the Eastern Tianshan, NW China: Implications for the Late Paleozoic Evolution of the North Tianshan Ocean

AbstractThe Kangurtag belt in the Eastern Tianshan, connecting the Dananhu Arc with the Central Tianshan Arc, contains diagnostic rocks of accretionary origin, and thus provides key information about the evolution of the North Tianshan Ocean. The Southern Kangurtag belt is composed of two types of mélange. Type I mélange consists of Enriched Mid‐Ocean Ridge Basalt‐type pillow basalts, draped by biohermal limestones and carbonate‐siliceous sediments of a slope facies, and siliceous argillites from a hemipelagic‐pelagic environment that together make up a seamount assemblage. In Type II mélange, Normal Mid‐Ocean Ridge Basalt and ribbon cherts were dismembered and entrained in a clastic matrix, showing a “block‐in‐matrix” structure. Detrital zircons of four sandstones from Devonian and Carboniferous strata within the mélanges have a predominant age population of 410–430 Ma and a distinct Proterozoic cluster around 1.4–1.6 Ga. The εHf(t) values of Phanerozoic zircons range from −25.1 to +8.6. Such age patterns, typical of the Central Tianshan Arc, and the Hf isotopic data indicate that these sedimentary successions were deposited on the northern margin of the Central Tianshan Arc. The youngest detrital zircon age of 317 Ma provides an upper limit for the time of formation of the Southern Kangurtag accretionary complex. Therefore, we suggest that the Southern Kangurtag belt comprises an accretionary complex that developed during southward subduction of the North Tianshan Ocean beneath the Central Tianshan Arc. This subduction began in the Early Ordovician and may have lasted until the Late Carboniferous–Permian.

Ziyang Fauna: A slope facies fossil lagerstätte of Early Ordovician in South China

“Great Ordovician Biodiversification Event” (GOBE) is one of the major biological innovation in Phanerozoic. Trilobite is one of the major taxa in Cambrian and Ordovician. In 1998, Adrain divided the global Ordovician trilobites into two groups, i.e. Ibex Fauna and Whiterock Fauna, and subsequently, the former was subdivided into Ibex I and Ibex II faunas in 2004. The Ordovician radiation of the trilobites was associated with temporal and spatial expansion of the Whiterock Fauna and contraction of the Ibex Fauna. Ibex Fauna is also known as the Cambrian Evolutionary Fauna, which originated at Cambrian, had high diversity and differentiation soon, decline rapidly in Early Ordovician, and went extinct in Late Ordovician. The Ibex I Fauna raised and flourished only in the Early Tremadocian, while Ibex II fauna declined after the Early Ordovician radiation. Whiterock Fauna is also called the Paleozoic Evolutionary Fauna, which appears in Early Ordovician and shows rapidly radiation and high diversification in Middle Ordovician, and then survived the Ordovician Mass Extinction, evolved into Silurian Fauna, some of them might develop and extend to Devonian, Carboniferous and Permian. In 2006, Zhiyi Zhou dubbed the Ordovician trilobite radiation in the South China Block, suggested that the time of onset and the environmental pattern of the Ordovician trilobite radiation may have varied geographically, and the major radiation was likely a long-sustained event that extended through about 25 Ma of the late Early-early Late Ordovician interval, coinciding with the worldwide expansion and domination of the Whiterock Fauna. So the taxa of Zhou’s Ibex Fauna were proportionally dominant over those of Whiterock Fauna. Ziyang fauna, an Early Ordovician fossil lagerstatte is discovered and reported from Ziyang County, Shaanxi Province. It is Floian (the late Early Ordovician) in age based on the occurrences of Tetragraptus approximatus Nicholson. The fossiliferous rocks are blackish-grey mud limestone and calcareous mudstone of possible slope facies in the Yangzte platform. The fauna is dominated by trilobites, accompanied with graptolites, brachiopods, arthropods and some trace fossils. With relatively high taxonomic diversity, the trilobites in this fauna mainly consist of Seleneceme , Niobe , Nileus , Ampyx , Degamella , Borogothus , Geragnostus and Dividuagnostus , etc assgigned to seven families, of them, Asaphidae, Nileidae, Agnostoidea, Peragnostoidea and Alsataspidiae belong to the Ibex Fauna, and Cyclopygidae and Raphiophoridae belong to Whiterock Fauna. Asaphidae and Nileidae are main components of Ibex I Fauna, and Agnostoidea, Peragnostoidea and Alsataspidiae are components of Ibex II Fauna respectively. Quantitatively, Alsataspidiae and Asaphidae account for more than 75% of the total fauna, Ibex I Fauna were predominent over Ibex II Fauna. Other associated fossils include graptolites of Tetragraptus approximatus , Tetragraptus bigsbyi , Tetragraptus quadribrachiatus , Tetragraptus sp., and Paratemnograptus sp., and brachiopods of Tritoechia , Lingulella , etc. The distinctive composition of Ziyang Fauna holds promise for providing insights into the evolution between Ibex and Whiterock faunas in South China.

Diagenetic controls on the isotopic composition of carbonate‐associated sulphate in the Permian Capitan Reef Complex, West Texas

AbstractLate Palaeozoic‐age strata from the Capitan Reef in west Texas show facies‐dependent heterogeneity in the sulphur isotopic composition of carbonate‐associated sulphate, which is trace sulphate incorporated into carbonate minerals that is often used to reconstruct the sulphur isotopic composition of ancient seawater. However, diagenetic pore fluid processes may influence the sulphur isotopic composition of carbonate‐associated sulphate. These processes variously modify the sulphur isotopic composition of incorporated sulphate from syndepositional seawater in shelf crest, outer shelf, shelf margin and slope depositional settings. This study used a new multicollector inductively‐coupled plasma mass spectrometry technique to determine the sulphur isotopic composition of samples of individual depositional and diagenetic textures. Carbonate rocks representing peritidal facies in the Yates and Tansill formations preserve the sulphur isotopic composition of Guadalupian seawater sulphate despite alteration of the carbon and oxygen isotopic compositions by meteoric and dolomitizing diagenetic processes. However, sulphur isotopic data indicate that limestones deposited in reef and slope facies in the Capitan and Bell Canyon formations largely incorporate sulphate from anoxic marine‐phreatic pore fluids isotopically modified from seawater by microbial sulphate reduction, despite generally preserving the carbon and oxygen isotopic compositions of Permian seawater. Some early and all late meteoric calcite cements have carbonate‐associated sulphate with a sulphur isotopic composition distinct from that of Permian seawater. Detailed petrographic and sedimentary context for carbonate‐associated sulphate analyses will allow for improved reconstructions of ancient seawater composition and diagenetic conditions in ancient carbonate platforms. The results of this study indicate that carbonate rocks that diagenetically stabilize in high‐energy environments without pore fluid sulphate gradients can provide a robust archive of ancient seawater's sulphur isotopic composition.

Upper Ordovician (Sandbian) conodonts from the Hoedongri Formation of western Jeongseon, Korea

Five sections in western Jeongseon-eastern Pyeongchang provide a relatively complete composite section of carbonate platform facies through the upper part of the Hoedongri Formation. A total of 29 samples (125 kg) yielded 669 relatively well-preserved conodont specimens, which are classified into 16 species representing 10 genera. The fauna is dominated by Panderodus gracilis (Branson and Mehl) (55%), which generally is believed to have been pelagic and eurytopic, and comprises characteristic elements of North China faunal affinities. Other taxa include Aphelognathus solidum Pei, Belodina sp., Eoligonodina prima (Branson and Mehl), Erismodus asymmetricus (Branson and Mehl), E. quadridactylus (Stauffer), Plectodina aculeata (Stauffer), P. alatheta An, Tasmanognathus careyi Burrett, and T. sishuiensis Zhang. This assemblage was assigned to the Tasmanognathus sishuiensis-Erismodus asymmetricus Zone herein. The interval of the zone is between the first and last occurrences of the eponymous species. The lower limit of the zone is unclear due to the absence of conodont data from underlying dolomitic limestone beds. The T. sishuiensis-E. asymmetricus Zone is the youngest conodont zone in the Taebaeksan Basin, Korea and was correlated to the ‘upper fauna’ of the Yeongheung Formation in Yeongwol, Korea, to the Belodina compressa-Microcoelodus symmetricus Zone in Shandong, to the T. sishuiensis-Erismodus typus Zone in Gansu, and to the T. sishuiensis Zone in the Ordos Basin, North China, representing the interior platform and slope facies of Sandbian age. Based on the recent recognition of the Aurilobodus serratus Zone and T. sishuiensis-E. asymmetricus Zone, the Hoedongri Formation was assigned to the Middle Ordovician to early Late Ordovician (Darriwilian, D3 to Sandbian, Sa2).

New understanding of the sedimentary model of Lower Cambrian Longwangmiao Formation reservoirs in the Sichuan Basin

Natural gas in the gas reservoirs of Lower Cambrian Longwangmiao Fm is mainly concentrated in the granular shoal facies in central Sichuan Basin, therefore the study on lithofacies paleogeography and the prediction on granular shoal distribution are the key to improving the success ratio of natural gas exploration in this area. In this paper, outcrop, drilling, seismic and logging data were studied comprehensively. Subsequently, the sedimentary facies and sedimentary model of Longwangmiao Formation in the Sichuan Basin and its adjacent areas were studied, and lithofacies paleogeographic maps were prepared based on previous research results. Besides, “three-shoal” sedimentary model of Longwangmiao Fm was established, the distribution range of three shoal belts was predicted and their development characteristics and main controls were analyzed, thereby pointing out the direction and field of natural gas exploration in the next stage. It is indicated from the research results that: first, the paleogeographic pattern of the Sichuan Basin in the Longwangmiao period was represented as a rimed carbonate rock platform, with the developments of sedimentary facies such as mixed tidal flat, restricted platform, open platform, platform margin and slope facies. Second, the “three-shoal” sedimentary model of Longwangmiao Fm was established. One of them is the granular shoal in platform margin facies and is distributed along the eastern margin of the Sichuan Basin. The other two are intra-platform granular shoals which were distributed on the east and west sides of lagoon subfacies, belonging to restricted platform facies and open platform facies, respectively. Third, the granular shoal of restricted platform facies is the main reservoir of Longwangmiao Fm gas reservoir in the Anyue Gas Field. It is distributed in central Sichuan, and its development is controlled by lagoons and Gaoshiti–Moxi Paleo-uplift. Fourth, the platform margin granular shoal is mainly distributed along the northern and eastern margins of the basin, and its development is controlled by the slope break of sedimentary paleogeomorphy. In conclusion, the granular shoal of restricted platform facies and shoal of platform margin facies are good reservoirs that they are the important fields of natural gas exploration. Furthermore, the new understanding to “three-shoal” sedimentary model and granular shoal distribution of Longwangmiao Fm is of great theoretical significance to the study on sedimentary systems of ancient strata.

Aptian carbonate platform development in the Southern Iberian Palaeomargin (Prebetic of Alicante, SE Spain)

The Aptian stratigraphic record of the Alicante region consists of: a rudist and coral-rich carbonate platform of earliest Aptian age (Llopis Formation), with a discontinuous siliciclastic member at its top; followed by late Early, to Late Aptian hemipelagic marls and marlstones (Almadich Formation); and then by renewed carbonate platform development of Late Aptian to earliest Albian age (Seguilí Formation). In the Llopis Formation, SW-dipping, massive clinoform beds of bioclastic debris are succeeded by flat-lying platform-top beds. The latter show a cyclically regressive stacking of biofacies, with rudist-dominated floatstone in their lower parts passing upwards to finer-grained, more sparsely fossiliferous bed tops with burrow mottling. Caprinid rudists, with originally almost wholly aragonitic shells, dominate the external platform-top facies, while more internal facies contain a mix of monopleurid, polyconitid and requieniid rudists, all with relatively slightly thicker development of the calcitic outer shell layer, together with caprinids. Biostratigraphic and carbon-isotope data link the termination of the Llopis platform with the onset of OAE1a. The carbonate platform of the Seguilí Formation again contains tabular platform-top beds showing repeated cyclic regression, with dense rudist and/or chondrodont floatstones overlain by sparser floatstones with wackestone matrix and secondarily filled burrows. But caprinids are now absent, while requieniids and polyconitids, some of large size, as well as radiolitids, all with thickened calcitic outer shell layers, accompany the tubular monopleurid, Mathesia, together with a greater development of Chondrodonta biofacies. The same overall pattern of biotic turnover from the Early, to the Late Aptian is confirmed in other parts of Iberia and contiguous regions. Moreover, Iberian platforms of late Early Aptian age outside the present study area reveal a transitional phase with an increasing proportion of polyconitids in the outer platform-top to upper slope facies at the expense of caprinids. The siliciclastic influx at the top of the Llopis Formation implies a climatic shift from arid, to relatively more humid/pluvial conditions through the mid-Early Aptian, as seen in several other Iberian sections. This climatic change was probably forced by the intensified greenhouse conditions at the onset of OAE1a. By contrast with these Iberian platforms, caprinids continued to dominate the outer platform-top zones of some central to southern Tethyan platforms until the close of the Early Aptian. This broad palaeolatitudinal differentiation of rudist associations within the Tethyan belt implies a climatic influence, whether exerted through thermal modulation of seawater pH and/or aragonite saturation, variation in nutrient flux, or any combination of these.

Response of Ostracods (Crustacea) to the Devonian F—F event: Evidence from the Yangdi and Nandong sections in Guangxi, South China

Abstract This study focuses on the Late Devonian ostracods and their responses to the F—F (Frasnian-Famennian) event based on abundant materials collected from the Yangdi and Nandong sections in South China. More than half (53%) of the benthic ostracod species disappeared at the F—F boundary in the Yangdi section of slope facies, whereas most benthic ostracod species survived the F—F crisis in the Nandong section of basinal facies, with the extinction of some pelagic entomozooidea. In view of a higher extinction rate (more than 65%) of ostracod species in the shallow-water Xikuangshan section, we deduce that ostracods in varied environments responded to the F—F event differently. Namely, from the carbonate platform, slope, to basinal settings, the extinction rate of benthic ostracods at the F—F transition decreased with increasing water depth. Nevertheless, the F—F event caused substantial loss among pelagic entomozooidea lineages. The extinction pattern of benthic and pelagic ostracods in F—F event in South China is similar to the situation in Europe. But the extinction rates of benthic ostracods in shallow water and slope in Europe seems to be higher than that in the same situation in South China. The abundance and diversity of ostracods decreased and some species even disappeared in anoxic environments across the F—F boundary in South China, when there were rapid changes in the palaeotemperature of surface sea water.

Internal Facies Architecture and Evolution History of Changxing Mouth-Bar Complex in the Changjiang (Yangtze) Delta, China

River-mouth bar is a fundamental element to constitute a delta system, and its internal facies architectures are vital to reconstruct delta evolution history and study high-resolution sequence stratigraphy. Changxing Island is a representative mouth-bar complex in the modern Changjiang Delta. Its vertical stacked strata produced by delta progradation were first studied by detailed facies analysis of core CX03, and then a general facies architecture and evolution history of mouth-bar complexes in the modern Changjiang Delta were discussed through a comparative study of 5 different cores and historical charts. A progradational deltaic sequence has generally a similar internal facies architecture, composed of massive muddy deposits of prodelta and delta-front slope facies at the bottom, sand-dominated deposits of delta-front platform, inter-bar channel and mouth-bar facies at the middle, and heterolithic deposits of tidal flat and saltmarsh facies at the top. There is a significant time lag for the initial formation of channel-mouth bar systems between Chongming and Jiuduansha Islands. Compared with subtidal flats, sedimentation rates on intertidal flats were highly accelerated, potentially resulting from weakening wave and tidal strength over extensive intertidal flats and increasing human embankment activities in the last few centuries. These findings provide some new clues to understand modern and ancient mouth-bar evolution and resultant sequence strata in tide-dominated deltas.

Gas Occurrence and Accumulation Characteristics of Cambrian–Ordovician Shales in the Tarim Basin, Northwest China

AbstractThe Tarim Basin is located in northwestern China and is the biggest basin in China with huge oil and gas resources. Especially the Lower to Middle Cambrian and Middle to Upper Ordovician possess the major marine source rocks in the Tarim Basin and have large shale gas resource potential. The Cambrian–Ordovician shales were mainly deposited in basin–slope facies with thicknesses between 30–180 m. For shales buried shallower than 4500 m, there is high organic matter abundance with TOC (total organic carbon) mainly between 1.0% and 6.0%, favorable organic matter of Type I and Type II, and high thermal maturity with RoE as 1.3%–2.75%. The mineral composition of these Cambrian–Ordovician shale samples is mainly quartz and carbonate minerals while the clay minerals content is mostly lower than 30%, because these samples include siliceous and calcareous shale and marlstone. The Cambrian and Ordovician shales are compacted with mean porosity of 4% and 3%, permeability of 0.0003×10–3–0.09×10–3 μm2 and 0.0002×10–3–0.11×10–3 μm2, and density of 2.30 g/m3 and 2.55 g/m3, respectively. The pores in the shale samples show good connectivity and are mainly mesopore in size. Different genetic types of pores can be observed such as intercrystal, intergranular, dissolved, organic matter and shrinkage joint. The reservoir bed properties are controlled by mineral composition and diagenesis. The maximum adsorption amount to methane of these shales is 1.15–7.36 cm3/g, with main affecting factors being organic matter abundance, porosity and thermal maturity. The accumulation characteristics of natural gas within these shales are jointly controlled by sedimentation, diagenesis, hydrocarbon generation conditions, reservoir bed properties and the occurrence process of natural gas. The natural gas underwent short‐distance migration and accumulation, in‐place accumulation in the early stage, and adjustment and modification in the later stage. Finally, the Yulin (well Y1) and Tazhong (well T1) areas are identified as the targets for shale gas exploration in the Tarim Basin.

Origin of facies zonation in microbial carbonate platform slopes: Clues from trace element and stable isotope geochemistry (Middle Triassic, Dolomites, Italy)

AbstractLimestones containing radiaxial fibrous cements were sampled along the southern slope of the late Anisian (Middle Triassic) Latemar carbonate platform in the Dolomites, northern Italy. The Latemar upper slopes comprise massive microbial boundstone, whereas lower slopes are made of clinostratified grainstone, rudstone and breccia. Samples are representative of a seawater column from near sea‐level to an aphotic zone at about 500 m water depth. Radiaxial fibrous cements were analyzed for carbon (δ13C) and oxygen (δ18O) stable isotopic composition, as well as major and trace element content, to shed light on the origin of the slope facies zonation. The δ13C vary between 1·7‰ and 2·3‰ (Vienna Pee‐Dee Belemnite), with lowest values at palaeo‐water depths between 70 m and 300 m. Radiaxial fibrous cements yielded seawater‐like rare earth element patterns with light rare earth element depletion (NdSN/YbSN ≈ 0·4), superchondritic yttrium/holmium ratios (≈55) and negative cerium anomalies. Cadmium reaches maximum values of ca 0·5 to 0·7 μg/g at palaeo‐water depths between 70 m and 300 m; barium contents (0·8 to 1·8 μg/g) increase linearly with depth. The downslope patterns of δ13C and cadmium suggest increased nutrient and organic matter contents at depths between ca 70 m and 300 m and point to an active biological pump. The peak in cadmium and the minimum of δ13C mark a zone of maximum organic matter respiration and high nutrient and organic matter availability. The base of this zone at ca 300 m depth corresponds with the transition from massive microbial boundstone to clinostratified grainstone, rudstone and breccia. The microbial boundstone facies apparently formed only in seawater enriched in organic matter, possibly because this organic matter sustained benthic microbial communities at Latemar. The base of slope microbialites on high‐relief microbial carbonate platforms may be a proxy for the depth to maximum respiration zones of Palaeozoic and Mesozoic periplatform basins.

Constraints on Paleoproterozoic atmospheric oxygen levels

The oxygenation of Earth's surface environment dramatically altered key biological and geochemical cycles and ultimately ushered in the rise of an ecologically diverse biosphere. However, atmospheric oxygen partial pressures (pO2) estimates for large swaths of the Precambrian remain intensely debated. Here we evaluate and explore the use of carbonate cerium (Ce) anomalies (Ce/Ce*) as a quantitative atmospheric pO2 proxy and provide estimates of Proterozoic pO2 using marine carbonates from a unique Precambrian carbonate succession-the Paleoproterozoic Pethei Group. In contrast to most previous work, we measure Ce/Ce* on marine carbonate precipitates that formed in situ across a depth gradient, building on previous detailed sedimentology and stratigraphy to constrain the paleo-depth of each sample. Measuring Ce/Ce* across a full platform to basin depth gradient, we found only minor depleted Ce anomalies restricted to the platform and upper slope facies. We combine these results with a Ce oxidation model to provide a quantitative constraint on atmospheric pO2 1.87 billion years ago (Ga). Our results suggest Paleoproterozoic atmospheric oxygen concentrations were low, near 0.1% of the present atmospheric level. This work provides another crucial line of empirical evidence that atmospheric oxygen levels returned to low concentrations following the Lomagundi Event, and remained low enough for large portions of the Proterozoic to have impacted the ecology of the earliest complex organisms.

Integrated Lower–Middle Ordovician graptolite and chitinozoan biostratigraphy of the Jiangnan Slope Region, South China

An integrated Early–Middle Ordovician graptolite and chitinozoan biostratigraphical study was carried out to improve the correlations between the two fossil groups in two sections in the Jiangnan Slope Region (deeper outer-shelf mud–sand–carbonate belt), South China. Six graptolite biozones and two chitinozoan biozones (including an undefined interval) were recognized. The chitinozoan Lagenochitina praepirum appears to be a stratigraphically useful species, despite being geographically restricted to South China, appearing at a level coincident with the first appearance of the index graptolite Levisograptus austrodentatus. Thus, the base of the Lagenochitina praepirum Biozone can also be regarded as the base of the Darriwilian Stage. The chitinozoan abundance in both sections is exceptionally low, and the assemblages do not contain the biostratigraphically important Belonechitina species. The distributions of both chitinozoans and graptolites are eco-dependent but show different onshore–offshore diversity trends. Chitinozoans are more diverse nearer to shore whereas graptolites are more diverse offshore, preferring slope facies. This may be caused by the different ecological preferences of chitinozoans and graptolites.

Crescentiella-microbial-cement microframeworks in the Upper Jurassic reefs of the Crimean Peninsula

In the Upper Jurassic reef successions of the Crimean Peninsula (Sudak and Jalta areas), the microencruster Crescentiella morronensis (Crescenti), microbialites, and multiple generations of cements, form microframeworks. They were observed in two stages of the carbonate platform evolution, in the Middle–Upper Oxfordian, and in the Upper Kimmeridgian–Tithonian. Generally, in both stages, the features of the microframeworks are similar and consist of densely packed Crescentiella associated with microbialites and branched colonies of the sclerosponge Neuropora lusitanica Termier. The difference between the occurrences of the two stages is the variable amount of nubecularid foraminifera and enigmatic tube-shaped structures forming the central cavities of Crescentiella. The Crescentiella-microbial-cement microframeworks formed under phreatic conditions in the upper slope and seaward marginal depositional settings where intensive synsedimentary cementation took place. They formed in the initial stages of long cycles of restoration and blooming of the reefs. The late Jurassic examples resemble the Permian algae-microbial-cement reefs as well as the Triassic Tubiphytes and cement crust-dominated reefs. Concurrently, all these examples formed a transitional facies zone between typical slope facies to shallow subtidal platform margin facies characterized by high taxonomic diversity of calcified sponges, corals, and microencrusters forming the principal part of the reefs.